Abstract

Swept-source optical coherence tomography angiography (SS-OCTA) is a promising new imaging modality for assessing retinal and choroidal vasculature. Faster scanning speed, large number of A-scan acquisition, and enhanced depth penetration has enhanced the detailed analysis of retinal layers. The authors discuss SS-OCTA features of a rare case of retinal arteriovenous malformation. Image analysis revealed the anomalous large-caliber vessels occupying up to the entire retinal thickness with associated echolucent changes in the inner retinal layers surrounding the retinal vessels, along with disruption of the outer retinal layers, including the inner/outer segments of photoreceptors beneath the large tortuous vessels outside the foveola in absence of any capillary nonperfusion areas or lack of significant macular edema. At the fovea, the outer retinal layers were intact due to a smaller caliber and less-deep extension of the anomalous vessels.

Abstract

Swept-source optical coherence tomography angiography (SS-OCTA) is a promising new imaging modality for assessing retinal and choroidal vasculature. Faster scanning speed, large number of A-scan acquisition, and enhanced depth penetration has enhanced the detailed analysis of retinal layers. The authors discuss SS-OCTA features of a rare case of retinal arteriovenous malformation. Image analysis revealed the anomalous large-caliber vessels occupying up to the entire retinal thickness with associated echolucent changes in the inner retinal layers surrounding the retinal vessels, along with disruption of the outer retinal layers, including the inner/outer segments of photoreceptors beneath the large tortuous vessels outside the foveola in absence of any capillary nonperfusion areas or lack of significant macular edema. At the fovea, the outer retinal layers were intact due to a smaller caliber and less-deep extension of the anomalous vessels.

[Ophthalmic Surg Lasers Imaging Retina. 2018;49:216–219.]

Introduction

Wyburn Mason syndrome, also known as Bonnet-Dechaumme-Blanc syndrome, was first described in 1943.1 This syndrome is sporadic in nature with the involvement of brain in 30% of cases followed by skin, kidney, bone, and the muscles. These malformations are due to an embryonic vascular mesoderm abnormality extending from the retina to the mesencephalon. The diagnosis is mainly based on clinical findings revealing dilated tortuous retinal vessels extending from the optic disc to the retinal periphery. Spectral-domain optical coherence tomography (SD-OCT) and other modalities are able to delineate the abnormal vascular and capillary patterns in these cases. A case of retinal arteriovenous malformation (AVM) with large-caliber vessels involving the entire thickness of retina was evaluated using swept-source OCT (SS-OCT)/OCT angiography (OCTA) for a better understanding of the morphological changes.

Case Description

A 24-year-old healthy male patient presented with history of low vision in the right eye for the past 3 months. There was no history of trauma/surgical intervention. Visual acuity (VA) was 20/60 and 20/20 in the right and left eyes, respectively, without any relative afferent pupillary defect. Both eyes anterior segment was unremarkable. The right eye fundus showed large dilated tortuous vascular channels in all four quadrants of the retina, the larger vascular loops were mainly seen in the superior half of macula along with medium-caliber vessels encroaching on to the superior and temporal aspect of fovea (Figure 1). Orbital ultrasonography did not reveal any malformation. The left eye was normal.

Figure 1.

Fundus photograph of right eye retinal arteriovenous malformation.

SS-OCT revealed that the larger anomalous vessels traversed the entire thickness of retina extending from internal limiting membrane (ILM) to the ellipsoid band (green arrows) above the retinal pigment epithelium (RPE) (Figure 2, top). The external limiting membrane (ELM), ellipsoid zone (EZ), and interdigitation zone seemed disrupted/involved, whereas the RPE was intact. The larger vessels also seemed to cause schisis (blue arrow) of the inner retinal layers sparing the ILM, which was draped over these vessels (Figure 2, top). The smaller-caliber anomalous vessels were limited to the inner retina without actual involvement of the outer retinal layers or the RPE, which only appeared interrupted in segments due to shadowing. There were no cystoid changes or any neurosensory detachment. At the fovea (Figure 2, bottom), the foveal contour was lost and the ILM appeared lifted and separated from the retinal layers due to adjacent anomalous vessels (red arrow) (central retinal thickness: 201 μm; subfoveal choroidal thickness: 281 μm). At the foveal center, the ELM, EZ, interdigitation zone, and RPE layer appeared intact. The near-normal anatomy at the fovea probably explained the VA of 20/60 in this eye. The anomalous vessels were clearly seen on the superficial retinal vascular segment on OCTA at the macula. These were passing through the area of the foveal avascular zone, which was not well-defined. The adjacent normal superficial vessels appeared a bit displaced (Figure 3, left). The anomalous vessels were also visualized in the deeper avascular retinal layers (Figures 3, middle and right). Capillary nonperfusion areas were not evident. The patient did not consent for central nervous system imaging and opted for regular follow-up, instead. The patient was counseled and kept under observation.

(Left) The anomalous vessels are seen on the superficial retinal vascular segment on optical coherence tomography angiography (OCTA). Foveal avascular zone is not well-defined. The adjacent normal superficial vessels appeared a bit displaced. Capillary nonperfusion areas are not seen. (Middle) The large anomalous arteriovenous malformation vessels are also visualized in deeper nonvascular segments of the outer retina on OCTA. (Right) Composite color-coded OCTA map of retinal vasculature showing the vasculature at various depths in different colors with orange representing most superficial plexus, green being at the level of the deep plexus and light blue at the level of the outer retina.

Discussion

The retinal AVMs are rare congenital unilateral anomalies that are usually asymptomatic and are categorized into three groups: Group 1 cases reveal retinal AVM with abnormal capillary beds between communicating channels in one quadrant. Group 2 cases have retinal AVM without any capillary interposition. Similarly, Group 3 cases have retinal AVM involving the entire retinal vasculature but without any capillary interposition.2 Our case satisfied Group 3 criteria.

These retinal AVMs are prenatally acquired due to certain genetic factors or insult to the growing embryo at specific time period. During embryological period, among the three vascular plexuses, the anterior plexus gives rise to the blood vessels of the retina and mid-brain. Retinal and mid-brain AVM are due to vascular dysgenesis of this anterior plexus. If the insult to the mesoderm occurs before seven weeks of gestation it is likely that both the organs will be affected (brain and retina), whereas tissue insult after 7 weeks leads to isolated retinal or mid-brain vascular malformation.3 The smaller malformation may have only minimal changes in the capillary network, whereas the one with larger malformation have direct arterial and venous communication without much alteration in interposing capillary bed leading to high blood flow. The histopathological examination of these vessels reveals no definite arterial/venous differentiation but rather a fibromuscular wall.4

The understanding of chorioretinal pathologies is increasing with the evolution of OCT from the time-domain to the newer SS-OCT, which possesses a high scanning speed (100,000 A-scans per second). With the earlier SD-OCT, the extent of retinal arteriovenous malformation has been described.4 The current case under discussion revealed following findings: 1) Large-caliber intraretinal vessels extending from the ILM to the RPE; 2) areas of schisis in inner retina with overlying separation of the ILM; 3) in areas where the entire thickness of retina was replaced by vessels, the EZ / photoreceptors were not well-appreciated; 4) lack of capillary nonperfusion areas in the superficial retinal vasculature; 5) lack of significant leakage from these large well-formed AVMs and thus an absence of macular edema.

The cause of low vision in these cases has been proposed to be compression of nerve fiber layers or photoreceptors disruption/damage due to high flow in the macular region along with recurrent macular edema. Gas described cystic changes in the inner nuclear and outer plexiform layer due to serous transudates.5–7 In our case, the vision was quite preserved despite the large AVMs being quite close to the fovea. This is perhaps due to preservation of the photoreceptors at the fovea and lack of macular edema. To conclude, SS-OCT and OCTA helps in achieving a better understanding of the morphological changes and flow dynamics in such cases.

(Left) The anomalous vessels are seen on the superficial retinal vascular segment on optical coherence tomography angiography (OCTA). Foveal avascular zone is not well-defined. The adjacent normal superficial vessels appeared a bit displaced. Capillary nonperfusion areas are not seen. (Middle) The large anomalous arteriovenous malformation vessels are also visualized in deeper nonvascular segments of the outer retina on OCTA. (Right) Composite color-coded OCTA map of retinal vasculature showing the vasculature at various depths in different colors with orange representing most superficial plexus, green being at the level of the deep plexus and light blue at the level of the outer retina.